1. Field of the Invention
The invention relates to a process for the flotation of contaminating material out of an aqueous fibrous suspension 1.
2. Description of Background and Relevant Information
Such processes are used for separating at least a part of the suspended contaminating particles out of a suspension containing fibrous material. In flotation, a foam or scum containing the material to be separated out is known to form. A typical use for such a process is the processing of an aqueous fibrous suspension made of printed used paper in which the ink particles have already separated from the fibers so that they can be floated off. The flotation process described here utilizes the differences between fibrous material and undesired solid particles in such a way that the fibrous material remains in the fiber suspension due to its hydrophilic nature whereas the mentioned material particles are hydrophobic and, therefore, move into the foam along with the air bubbles. Aside from the ink particles, there is a multitude of other materials that are hydrophobic and which, therefore, can be separated from the fibrous material by means of flotation. Such materials are, in particular, adhesives, fine plastic particles, and, probably resins as well. When fibers are to be separated from contaminants by the process of flotation. i.e., not all solid particles are to be sorted out, it is called selective flotation. The term xe2x80x9cflotation de-inking,xe2x80x9d which is also used, is normally used not only for the removal of ink particles, but also more generally for the selective flotation of contaminating particles out of fibrous suspensions.
The prior art relating to the process of flotation for fibrous suspensions is already very advanced. Therefore. there are solutions which are appropriate for removing a large part of the solid particles by flotation.
In EP 0 798 416 A1, a flotation process for a fibrous suspension is described. This suspension is mixed with a gas, e.g., air, and introduced directly into an existing suspension already in a flotation container. The solid matter to be separated from the suspension reaches the flotation foam with the aid of air bubbles. For the purpose of improving this process, this publication suggests adding a liquid, which can be, e.g., water with or without chemicals, in a very specific way to the flotation foam thus formed.
Since flotation devices are relatively expensive in their production and use, it is an understandable goal to improve their effectiveness or to reduce the expense necessary for achieving the same results.
The invention therefore provides for a flotation process in which good purification results can be achieved while simultaneously keeping the installation and operating expenses as low as possible.
A process for the flotation of contaminates out of an aqueous fibrous suspension using a foam, the process includes creating the foam, introducing the fibrous suspension into the foam, transferring contaminates from the fibrous suspension to the foam, and removing some portion of the fibrous suspension from the foam as an accepted stock.
Through the process according to the invention, the optimal utilization of the means employed is possible. In particular, the gas used for the flotation, especially air, can be utilized optimally. The unaerated fibrous suspension is added directly into the rising foam or at a location from which it can, in opposition to the rising direction of the foam, arrive immediately therein. Therefore, it strikes a preexisting air bubble composite with imbedded liquid channels through which the suspension sinks down. Thus, it achieves optimal contact to the bubble surfaces. Additionally, it is advantageous that the principle of inverse flow can be realized in this manner. This means that the xe2x80x9cdirtiestxe2x80x9d suspension comes into contact with the already heavily loaded air bubbles, becomes increasingly cleaner on its way down where, therefore, the air bubbles encountered are carrying less dirt.